The goal of this project is to assess the efficacy of magnesium oxide on the reversal of elastic fiber calcification and clinical lesions of PXE. To achieve this goal, this proposal outlines three specific aims. (1) The primary endpoint is to assess the reversal of elastic fiber calcification through skin biopsies. A blinded dermatopathologist will grade skin biopsies on the density of Von Kossa staining. The amount of calcification of elastic fibers will be determined by assessing von Kossa staining per unit area of dermis. (2) The secondary endpoint is to assess the reversal of clinical skin lesions through investigator evaluation and clinical photographs. (3) The final aim is to assess the rate of disease progression through ophthalmologic examinations. The study consists of two parts. The first part is a year-long, double-blind, placebo-controlled study. Part two is an open-label, year-long study. In Part 1, qualified subjects will be randomized to receive either magnesium oxide supplements, or placebo, in a 1:1 ratio for the first 12 months. The starting dose will be 1000 mg daily, and depending on tolerability, doses will be decreased. Baseline evaluations will be comprised of blood chemistry panel, serum magnesium, calcium and phosphorus levels, complete blood count testing, urinalysis, clinical evaluations, punch biopsy of target lesion, ophthalmologic evaluations, photographs of skin lesions, electrocardiogram, and bone densitometry. Upon completion of the first year, barring any safety concerns, all subjects will be administered magnesium supplements for up to one additional year. The same evaluations/procedures will be conducted during Part 2.